198 SOME CURIOSITIES OF VISION. 



actually appear, we can only do so with the greatest difficulty ; some- 

 times, indeed, not at all, unless with the assistance of some specially 

 contrived artifice. 



I propose to discuss to-night a few of the less familiar vagaries of 

 the visual organs, and will do my best to assist in the illustration of 

 them. But it will be my part merely to provide the apparatus for the 

 experiments; the experiments must themselves be carried out by each 

 of you individually. Some of them will, I am afraid, be found rather 

 difficult; success will depend mainly upon your power of laying aside 

 habit and prejudice and giving close attention to your visual sensations. 

 I hardly dare to hope that everyone present will observe all the peculi- 

 arities and defects which it is intended to demonstrate, but in case of 

 failure I generally find that there is a comfortable tendency to attribute 

 it not to any deficiency in the observer's power of concentrating his 

 attention, but to the fact that his eyes are not as other men's, and are 

 free from the particular defect which it is desired to bring into promi- 

 nence. Of course, anyone is welcome to such an entirely satisfactory 

 opinion. 



Among the most annoying of the eccentricities which characterize the 

 sense of vision is that known as the persistence of impressions. The 

 sensation of sight which is produced by an illuminated object does not 

 cease at the moment when the exciting cause is removed or changed 

 in position, but continues for a period which is generally said to be about 

 one-tenth of a second, but may sometimes be much more or less. It is 

 for this reason that we can not see the details of anything which is in 

 rapid motion, but only an indistinct blur, resulting from the confusion 

 of successive impressions." When I turn this disk, which is painted in 

 black and white sectors, you soon lose sight of the divisions, and if 

 the speed is high enough the whole surface appears to be of a uniformly 

 gray hue. If we illuminate the rotating disk by a properly timed series 

 of electric flashes, it looks as if it were at rest, and in spite of the inter- 

 mittent nature of the light, the black and white sectors are seen quite 

 continuously, though as a matter of fact the intervals of darkness are 

 very much longer than those of illumination. 



The persistent impressions which we have been discussing are often 

 spoken of as positive after-images. 



There is one very remarkable phenomenon accompanying the forma- 

 tion of positive after-images, especially those following brief illumina- 

 tion, which seems, until comparatively recent times, to have entirely 

 escaped the notice of the most acute observers. It was first observed 

 accidentally by Prof. C. A. Young, when he was experimenting with a 

 large electrical machine which had been newly acquired for his labora- 

 tory. He noticed that when a powerful Leyden jar discharge took place 

 in a darkened room, any conspicuous object was seen twice at least, 

 with an interval of a trifle less than a quarter of a second, the first 

 time vividly, the second time faintly. Often it was seen a third time, 



